Field of the Invention
The invention lies in the field of printing technology. More specifically, the invention pertains to a method of extracting a printing plate from a plate cylinder in a printing unit.
Printing plates are typically thin metal foils formed with a surface that defines an image to be printed. The printing plate is clamped onto a plate cylinder that is rotatably mounted in a printing unit. After the plate has been inked, the printing ink is transferred from the image surface of the printing plate onto the material to be printed in the rotary printing machine. In offset printing, for instance, the print image is transferred from the printing plate onto a rubber blanket and from the rubber blanket onto paper or another material.
The useful surface of the printing plate extends from its trailing edge to its leading edge. In modern plate cylinders, the gap between the leading and trailing edges of the plate are made increasingly narrower, so as to limit the extent of the non-print area on the plate cylinder.
We described in U.S. Pat. No. 5,284,093 a plate cylinder with an open passage in which a plate locking mechanism is disposed. The leading and trailing edges, i.e., the edge portions, of the printing plate are held in a common gap that is formed between the locking mechanism and one wall of the passage in the plate cylinder. After the locking mechanism is closed, the gap between the edge portions of the printing plate is very narrow.
One problem associated with the narrowing of the plate gap is that it becomes more difficult for automatic plate changers to remove a spent printing plate. Such an automatic plate changer is described, for instance, in U.S. Pat. No. 5,390,603 and a magazine and plate change system is described in U.S. Pat. Nos. 5,443,006; 5,495,805; and 5,537,926. In those systems, old printing plates are automatically removed from the plate cylinder and stored in a magazine. A new plate is automatically dispensed from the magazine and clamped into the plate cylinder for starting a new print job.
While those prior systems operate quite reliably in general, they were originally designed with much larger plate gaps in mind. With the decreased gap, however, difficulties have arisen with regard to the plate change automation, because the target gap for the plate change mechanism is decreased so far that a reliable xe2x80x9chitxe2x80x9d cannot always be ensured and the edge of the plate may become caught during the removal with the prior art systems.
It is accordingly an object of the invention to provide a method of extracting a printing plate from the plate cylinder of a printing unit, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which is enabled to reliably change a plate with a very small target defined by the narrow plate gap.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of extracting a printing plate from a plate cylinder rotatably mounted in a printing unit, wherein a trailing edge and a leading edge of the printing plate are retained by a locking member in a plate gap of the plate cylinder. The method comprises the following steps:
releasing the trailing edge of the printing plate from the locking member of the plate cylinder;
plastically deforming the printing plate at the trailing edge; and
peeling the printing plate away and removing the printing plate from the plate cylinder.
In accordance with an added feature of the invention, the releasing step comprises axially displacing a lockup bar within an open passage of the plate cylinder and projecting a bearing surface of the lockup bar radially beyond a peripheral surface of the plate cylinder.
In accordance with an additional feature of the invention, the deforming step comprises forcing the printing plate towards the peripheral surface of the plate cylinder in a vicinity of the lockup bar projecting beyond the peripheral surface.
In accordance with another feature of the invention, the printing plate is biased against the peripheral surface of the plate cylinder with a tucker bar of an automatic plate change mechanism of the printing unit.
In accordance with a further feature of the invention, the peeling step comprises placing a guide fork of an automatic plate change mechanism in the vicinity of the plastically deformed trailing edge, rotating the plate cylinder to force the trailing edge to run onto the guide fork, and deflecting the printing plate away from the plate cylinder while continuing the rotation of the plate cylinder.
The invention is most applicable in the context of narrow plate gaps, i.e., where the leading and trailing edges of the printing plate are locked with a plate gap of less than 5 mm, and preferably less than 1 mm. In the preferred embodiments, the invention ensures that the printing plate is reliably xe2x80x9chitxe2x80x9d by the automatic plate changer. This ensures, for example, that automatic plate changes can occur reliably with the narrow gap cylinders used with the seamless rubber blanket.
In accordance with again an added feature of the invention, the guide fork is pivoted to cause its tip to engage the leading edge of the printing plate (the edge that is removed last from the cylinder) and move the leading edge away from the locking member. The translational movement of the guide fork thus essentially forces the leading edge away from the plate gap.
With the above and other objects in view there is provided a further method of extracting a printing plate, which comprises:
after the trailing edge of the printing plate has been released from the locking member, removing the printing plate from the plate cylinder with a guide fork of an automatic plate change mechanism, by rotating the plate cylinder and causing the printing plate to unravel from the plate cylinder; and
prior to the completed removal of the printing plate, unbending the bent-over leading edge of the printing plate to substantially flatten the leading edge. Flattening the leading edge ensures that the edge portion will come free from the plate cylinder and that the old printing plate can be properly handled in the automatic plate changer.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method of automatically extracting a printing plate from the plate cylinder of a printing unit, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.